A nuclear reactor power plant generates electric power as a result of the nuclear fission of radioactive materials contained within the nuclear reactor. Due to the volatility of this nuclear reaction, nuclear reactor power plants are designed in a manner to assure that the health and safety of the public is maintained.
In conventional nuclear reactors, the radioactive material used for generating electric power is nuclear fuel. The nuclear fuel is depleted, i.e., spent, over the life of the fuel cycle. The nuclear fuel is not reprocessed and therefore, the spent fuel is removed at periodic intervals from the nuclear reactor. Even after removal, the spent fuel continues to generate intense heat, called “decay heat,” and remains radioactive. Decay heat naturally decreases over time at an exponential rate, but still generates enough energy to require water cooling for several years. Thus, a safe storage facility is needed to receive and store the spent fuel. In nuclear reactor power plants, such as small modular reactors and other pressurized water reactors, a spent fuel pool is provided as a storage facility for the spent fuel following its removal from the reactor. The spent fuel pool is typically constructed of concrete and contains a level of water that is sufficient in order to maintain the nuclear fuel immersed underwater. The spent fuel pool is typically at least 40 feet deep. The quality of the water is also controlled and monitored to prevent fuel degradation in the spent fuel pool. Further, the water is continuously cooled to remove the heat which is produced by the spent fuel in the pool.
A typical nuclear reactor power plant includes an active spent fuel pool cooling system which is designed for and capable of removing decay heat generated by stored spent fuel from the water in the spent fuel pool. “Active” cooling systems include those which require alternating current electric power to operate pumps or valves in order to achieve the desired cooling function. Removal of the decay heat is necessary to maintain the spent fuel pool water temperature within acceptable regulatory limits and prevent unwanted boiling of the water in the spent fuel pool. In some pressurized water reactors, such as the AP1000 design which includes Westinghouse's Passive Core Cooling System, the spent fuel pool cooling system is a non-safety-related system. In other pressurized water reactor designs, such as non-AP1000 designs, the spent fuel pool cooling system is a safety-related system.
The active spent fuel pool cooling system typically includes a spent fuel pool pump to circulate high temperature water from the spent fuel pool and through a heat exchanger to cool the water. The cooled water is then returned to the spent fuel pool. The spent fuel pool cooling system can include two mechanical trains of equipment. Each train having one spent fuel pool pump, one spent fuel pool heat exchanger, one spent fuel pool demineralizer and one spent fuel pool filter. The two trains of equipment can share common suction and discharge headers. In addition, the spent fuel pool cooling system includes the piping, valves and instrumentation necessary for system operation. Typically, one train is continuously cooling and purifying the spent fuel pool while the other train is available for water transfers, in-containment refueling water storage tank purification, or alignment as a backup to the operating train of equipment.
FIG. 1 shows an active spent fuel pool cooling (SFPC) system 10 during its normal operation in accordance with the prior art. The SFPC 10 includes a spent fuel pool 5. The spent fuel pool 5 contains a level of water 16 at an elevated temperature as a result of the decay heat generated by the spent fuel (not shown) that is transferred from the nuclear reactor (not shown) into the spent fuel pool 5. The SFPC system 10 includes trains A and B. Trains A and B are employed to cool the water in the spent fuel pool 5. As previously described, it is typical to operate one of train A or train B to continuously cool and purify the spent fuel pool 5 while the other train is available as a back-up. Each of trains A and B include a SFPC pump 25, a SFPC demineralizer and filter system 45. Trains A and B share a common suction header 20 and a common discharge header 50. In each of trains A and B, water exits the spent fuel pool 5 through the suction header 20 and is pumped through the SFPC pump 25 to the SFPC heat exchanger 30. In the SFPC heat exchanger 30, a flow line 40 passes water from the component cooling water system (CCWS) (not shown) through the SFPC heat exchanger 30 and back to the CCWS. The heat from the water entering the SFPC heat exchanger 30 (from the spent fuel pool 5) is transferred to the water provided by the flow line 40 and is returned back to the CCWS through the flow line 40. Cooled water exits the SFPC heat exchanger 30 and passes through the SFPC demineralizer and filter system 45 positioned downstream of the SFPC heat exchanger 30. Purified, cooled water exits the demineralizer and filter system 45, is transported through the common discharge header 50, and is returned to the spent fuel pool 5.
In addition to the active SFPC system shown in FIG. 1, it is also known in the art to employ passive designs to mitigate accident events in a nuclear reactor without operator intervention or off-site power. These passive designs emphasize safety features that rely on natural forces, such as pressurized gas, gravity flow, natural circulation flow, and convection, and do not rely on active components (such as, pumps, fans or diesel generators). Further, passive systems are designed to function without safety grade support systems (such as, AC power, component cooling water, service water, and HVAC). A passive spent fuel pool cooling system can be designed such that the primary means for spent fuel protection is provided by passive means and relies on the boiling of the spent fuel pool water inventory to remove decay heat.
For example, if a complete loss or failure of an active spent fuel pool cooling system is assumed, spent fuel cooling can be provided by the heat capacity of the water in the spent fuel pool. The decay heat of the spent fuel is transferred to the water in the pool and, after some period of time, causes the water to boil. The boiling action of the pool water produces non-radioactive steam, which transfers the decay heat energy to the atmosphere. After a specific period of time, additional water will need to be added to the SFP to makeup for the loss of inventory due to boiling. Water make-up can be provided to the spent fuel pool by alternate means to maintain the pool water level above the top of the spent fuel and boiling of the pool water can continue to provide for the removal of decay heat. Boiling of the spent fuel pool water releases large quantities of steam into the fuel handling area. The steam mixes with air in the fuel handling area to form a steam/air mixture which is then passively vented through an engineered relief panel to the atmosphere to reduce the temperature in the fuel handling area.
The boil-off rate of the spent fuel pool water is highly dependent on the decay heat generated by the fuel in the pool. The amount of decay heat generated depends on how recently fuel has been offloaded into the spent fuel pool. During the first 72 hours of a loss of cooling event, water is typically supplied from safety-related sources, such as the spent fuel pool inventory, water stored in the cask wash-down pit, and water from the fuel transfer canal. If additional makeup water is required beyond 72 hours, water from the passive containment cooling system ancillary water storage tank can be provided to the spent fuel pool.
The invention provides an alternate passive spent fuel cooling system and method that is employed to remove decay heat generated by the spent fuel in the event of a loss of onsite and offsite power wherein the active spent fuel pool cooling system is not available to cool the spent fuel pool.